Ethanol production presents four challenges that must be met in order to economically produce ethanol useful as a fuel additive. First, there must be an effective system so that primary stripping of ethanol/water from stillage (beer) can be accomplished and energy effective rectification of the ethanol/water mixture can be made. Second, an effective system for dehydrating the rectified ethanol/water product must be developed that integrates with the product distillation system and also is integrated in the energy management of that system. Third, an energy efficient system of de-watering the stillage to the maximum level must also be integrated into the overall system. Forth, the propensity for the stillage to foul surfaces in distillation and evaporation must be controlled to limit the time and expense of cleaning the system. Additionally, there is a need to limit energy usage in the dryer and thermal oxidizer which, are part of the system to recover dried distillers grains and soluble solids.
Diminishing world supplies and availability of crude oil as well as sporadic regional shortfalls of gasoline for motor fuel have created considerable incentive for the development and use of alternative fuels. Furthermore, environmental concerns have required use of additives which aid in oxygenation of the motor fuels. These additives have created concerns of their own for environmental damage. Ethanol is gaining wide popularity as a fuel additive capable of addressing these concerns, particularly when mixed with gasoline to form a mixture known as gasohol. Gasohol may contain up to about 10 vol. % ethanol, without modifications to presently used automobile engines being required, thereby extending the volume of motor fuel availability by a like percentage.
The source of the ethanol used in gasohol is derived primarily from the fermentation of mash, usually from corn or wheat or other grain. Natural fermentation is able to produce an ethanol-water product mixture containing, at the most, about 12 vol. % ethanol. This mixture may easily be concentrated by distillation to about 95% ethanol. Higher concentrations of ethanol, however, as required in gasohol are obtained only by expenditures of great amounts of energy and great difficulty due to the formation of an ethanol-water azeotrope at about the 95% ethanol concentration. A means of achieving the greater than 95% ethanol concentration without 1) such a great expenditure of energy or 2) loss of the used energy would thus be extremely valuable. Such schemes have been employed in the past to recover heat from azeotropic distillation employing tertiary entrainers such as benzene (U.S. Pat. Nos. 4,372,822, 4,422,903 and 5,035,776). Others earlier had considered the option of using heat from the stripping/rectifying column to heat an azeotropic distillation (U.S. Pat. Nos. 1,860,554 and 4,217,178). Additionally, one invention considered generating steam from the heat in overhead vapors of the azeotropic distillation (U.S. Pat. No. 4,161,429) and another used mechanical vapor recompression of the overhead vapors to recover heat in the fashion of a heat pump for heating the azeotropic distillation column(s) (U.S. Pat. No. 5,294,304).
Recent industrial practice includes the many known adsorptive separation processes known in the art for possible application to the separation of ethanol from water, but do so without attention to the heat that can be recovered from the process. The adsorption method is employed in many ways and of itself is a significant improvement for energy usage and environmental impact over azeotropic entrainers and azeotropic distillation. To this date however, little has been done to further improve heat recoveries for the adsorptive dehydration operation.
For general background of the art, it is well-known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon species from mixtures thereof. The separation of normal paraffins from branched chain paraffins, for example, can be accomplished by using a type A zeolite which has pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat. Nos. 3,265,750 and 3,510,423, for example, disclose processes in which large pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Pat. No. 3,114,782, for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267 a particular zeolite is used to separate specific alkyl-substituted naphthalenes. In processes described in U.S. Pat. Nos. 3,558,732, 3,686, 342 and 3,997,620, adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene over the other xylene isomers. In the last mentioned processes the adsorbents used are para-xylene selective; para-xylene is selectively adsorbed and recovered as an extract component while the rest of the xylenes and ethylbenzenes are all relatively unadsorbed with respect to para-xylene and are recovered as raffinate components. Also, in the last mentioned processes the adsorption and desorption may be continuously carried out in a simulated moving bed countercurrent flow system, the operating principles and sequence of which are described in U.S. Pat. No. 2,985,589.
The process of adsorption within the Zeolyte bed depends on two factors. The first is a basic affinity of the Zeolyte substrate for binding molecules of vapor in a condensed/adsorbed state. The second is a property of the Zeolyte that preferentially binds water rather than ethanol. The vapor, comprised of ethanol and water, is passed through the bed of type 3A Zeolyte. The water molecules diffuse into the 3 angstrom pores of the Zeolyte adsorbing onto the surface substrate. Since the ethanol molecules are much larger, they pass through the bed adsorbing as a very minor fraction on the exterior of the Zeolyte pellets.
Water adsorbing onto the Zeolyte surface largely forms a monolayer of “condensed” film. This releases energy into the bed and vapor passing through the bed. This energy is a combination of latent heat of vaporization/condensation for the water and a surface specific energy that results from the formation of the monolayer of water molecules on the substrate surface. During adsorption the bed increases in temperature and retains a large portion of this energy. During desorption the bed decreases in temperature as this heat is released with the evaporation of water from the Zeolyte.
Another problem presented in the production of ethanol is the removal of solids from the production stream. In the production of fuel alcohol from plant materials, the biomass is mixed with hot water to produce a wort, which is fermented until the final alcohol level is reached. The fermented contents are then typically discharged as a slurry to the beer well and from there to the beer still where the alcohol is removed by distillation. The remainder, after distillation, is known as the still bottoms or stillage, and consists of a large amount of water together with the spent solids.
Stillage in general has a complex composition, which in the case of corn feed stocks, includes the non-fermented fibers from the hull and tipcap of the corn kernel, as well as, particles of the corn germ with high oil content, oil and other lipids, the non-fermented portions of the corn kernel such as gluten, any residual unreacted starch, solubles such as proteins and enzymes, and the byproducts and residue of fermentation including dead yeast cells. The particle sizes range widely from broken parts of kernels 1-2 millimeters in size, down to fines in the under 10 micron range. Typically, stillage is dewatered to produce animal feeds rich in protein. This feed-production process has added benefit of reducing waste disposal costs from the alcohol production. It also has the very important benefit of providing a rich protein source to cattle not derived from reprocessed cattle carcasses (an important concern for transmission of damaging prions).
A conventional process for handling stillage, currently used in typical dry mill ethanol plants has aqueous solids, such as whole stillage from corn, flow from a distillation column to a solid bowl decanter centrifuge which separates the feed stream according to density into cake (the “heavier” substances), and thin stillage (the lighter substances). Since most corn solids are heavier than water, the cake contains most of the solids. The thin stillage typically has 8 to 15% solids of which about 10% or more are suspended insoluble solids, the remainder being dissolved solids including proteins, acids, unreacted sugars, and others. The suspended solids in the thin stillage are predominately fines but there is not a sharp cutoff since some larger particles are subject to carry-over with the liquid. Thin stillage is typically accumulated in a holding tank, from which typically 30-60% is recirculated as “backset” to the cooking and fermentation stages to provide nutrients and to reduce the fresh water requirements. The remainder of the thin stillage is sent to the evaporator which concentrates the solids to a syrup of typically 30-35% solids in dry mill plants. Wet mill plants, which do not have such a load of insoluble solids can achieve a syrup concentration of 50%. This syrup is added to the cake and the combined stream is, typically, sent to the dryer (not shown) to be dried to about 10-11% moisture.
The dewatering machinery which are generally most effective at producing high dry solids content, such as screen centrifuges and screw presses, have not proven feasible with corn stillage. Indeed, corn stillage and stillage from other grain fermentation has proven to be too fine and sticky for most separation devices. The typical industry practice has been to dewater such stillage using a solid bowl decanter centrifuge which is very functional, but which typically only produce cake solids content in the 30-35% range, in addition to having high electricity usage and high maintenance costs. However, up till now, the only way to improve performance of thin stillage evaporation has been to accomplish the most complete centrifugation of the stillage.
Numerous methods of overcoming this situation have been reported, such as separating most of the solids from the beer liquid prior to distillation so as to permit use of a screw press as described by B. J. Low in “The Efficient Production of High Quality Distiller's Dark Grains Using Stored Dehydration Process Technology.” The separation step is followed by dewatering in a screw press to a solids content of 50-54%, and then by drying in a special dryer. However, the presence of the alcohol at the separation step greatly complicates the drying process, requiring special closed-cycle dryers which are costly to purchase and expensive to maintain, as well as necessitating an alcohol vapor recovery system.
In some such ethanol production processes, such as in the production of ethanol from citrus residue as described in U.S. Pat. No. 4,952,504 issued to Pavilon, highly effective dewatering machinery such as screen centrifuges and screw presses (yielding dry-solids content typically 35-50% or higher) can be used to efficiently dewater solids filtered from the wort prior to fermentation. In fermentation from grains such as corn, however, this dewatering from the wort stage has the disadvantage of reducing the final alcohol yield.
U.S. Pat. No. 4,552,775, issued to Baeling, discloses a method for dewatering the stillage from a unique fermentation process which produces stillage of 20-30% dry substance (compared to the conventional corn fermentation which produces a stillage in the 5-12% solids range). This high solids stillage is combined with sufficient recycled dry product to obtain a 50-70% dry substance content which is then pelletized before drying in a through air dryer of special design. This method has the disadvantage that when applied to conventional stillages of 5-12% solids, the required recycle rate becomes very large, increasing the size and expense of the dryer.
A significant need remains for an improved, efficient and cost-effective method and apparatus to dewater conventional grain stillage, for the fuel alcohol industry.
The production of gasohol by the blending of fuel grade ethanol with gasoline has the potential for helping meet energy needs. Alcohol blends with gasoline require 99.35 percent alcohol. To make effective use of ethanol as a substitute fuel the energy consumed to make the fuel grade alcohol must be less than the energy obtained from ethanol (84,090 Btu/gal or 7120 cal/g).
The conventional method to concentrate an aqueous solution of ethanol involves two steps: first, a dilute ethanol-water mixture (6-12 percent ethanol) is distilled to about 95 percent; next, the solution of step one is azeotropically distilled to anhydrous alcohol having a concentration of about 99.8 percent. Distillation energy requirements are composed of the steam required for the main distillation step producing azeotropic ethanol and that required for breaking the azeotrope and producing essentially anhydrous ethanol. The energy for the first step depends more on the feed ethanol concentration than any other factor and this energy represents the minimum practical energy usage for a plant. Simple (non-azeotropic) distillation is limited with regard to ethanol-enrichment because the alcohol-water mixture forms a constant boiling azeotrope at 95.6 percent ethanol. One complication at this upper end is an inflection in the vapor-liquid equilibrium relationship, which upon closer approach to the azeotropic composition requires a considerable increase in the number of distillation trays required and the height of the column. The energy required for azeotropic distillation is typically recovered for use in preheating and to offset heat requirements in the main distillation. An example of this is U.S. Pat. No. 4,422,903. This patent teaches the art of constructing a double effect stripping/rectification column and recovering heat from azeotropic distillation to one of the two stripping/rectification columns.
The theoretical amount of energy expended to distill ethanol from 5 to 100 percent calculated by balancing heat input into the system and heat lost is about 3420 cal/g. In industrial practice, the actual energy expended during distillation is lower than theoretical due to the inclusion of various heat recovery systems. The reported loss of the fuel value to distill from 10 percent to 95 percent ethanol in industrial practice is about 13-21 percent; the loss of fuel value to concentrate from 95 to 100 percent by azeotropic distillation with benzene is an additional 7-11 percent. Overall expenditure is about 1400-2400 cal/g. The capital cost to produce 100 percent ethanol with an expenditure of only about 1400 cal/g is nearly double that of a distillation plant producing 95 percent ethanol due to the inclusion of azeotropic distillation equipment and advanced design heat recovery systems.
Several alternate approaches to obtain anhydrous ethanol which eliminate the energy costly azeotropic distillation have been suggested. These include dehydrating ethanol with such materials as gypsum, calcium chloride and lime, molecular sieves, biomass materials or the like, or solvent extraction. One technique involves the use of sorbents to selectively adsorb water from an ethanol-water mix. In the Purdue process (Chemical Engineering, Vol. 87, p. 103, Nov. 17, 1980), ethanol-rich vapors (80-92 percent ethanol) leaving a first stage distillation at a temperature of about 78 [deg]-80 [deg] C. are passed directly onto a column of cornmeal to adsorb water and obtain anhydrous ethanol. After the column is saturated, the cornmeal is regenerated by passing hot (90. degree.-120 [deg] C.) air over it; simultaneously, a second previously regenerated column is brought into operation. Overall energy expenditure for the distillation and sorption processes including the distillation step is about 1000 cal/g. The process is used in a modified fashion industrially in which corn grits and carbon dioxide are substituted for cornmeal and air.
The most accepted approach to dehydration now used industrially is to use type 3A Zeolyte molecular sieve adsorption. Typically a two bed system is used in which one bed receives a flow of azeotropic ethanol for dehydration and the other undergoes regeneration. The beds are operated in a vapor phase pressure swing approach. The dehydration takes place at an elevated pressure while the regeneration takes place under vacuum. Typically overhead azeotropic ethanol from distillation is condensed, pumped to a vaporizer to elevate the dehydration pressure then recondensed after dehydration. Under this process configuration the ethanol is condensed twice without recovery. Another industrially applied technique is to supply the azeotropic ethanol directly from the distillation column without first condensing and only condense after dehydration. In this case the ethanol is condensed once without recovery.
Thus a system that effectively reuses energy, effectively removes insoluble solids prior to evaporation, dewaters solids using waste heat, reduces the rate of fouling in distillation and on heating surfaces, and uses non-azeotropic methods of ethanol dehydration in which energy is further recovered to the process would be desirable.